Water on a Nepal trek: bottles, filters and tablets compared

For a Nepal trek, the safest and lightest water plan is to treat every drink in two steps or to drink water that a lodge has boiled: a microfilter or a boiled-water refill for bacteria and protozoa, plus a chemical step (chlorine dioxide tablets are the usual choice) for viruses. No single method is perfect: filters miss viruses, chlorine and iodine are weak against Cryptosporidium, and boiling needs fuel. The rest of this guide explains what each method does, based on the CDC Yellow Book chapter on water disinfection for travellers, and then shows how to choose for a teahouse trek, a camping trek and a short hike. It also covers bottles, plastic waste and the habits that cause most stomach problems on the trail.

A note on scope: we are explaining equipment, not giving medical advice. For illness, medication and vaccines, see the CDC Yellow Book and your own doctor or travel clinic. Our altitude sickness basics guide covers hydration and altitude.

What the CDC says about each method

The CDC Yellow Book groups travellers' water treatment into heat, filtration and chemical disinfection, and says the best choice depends on the water, the pathogens and what you can carry.

Boiling. The CDC says to boil water for a full minute, which allows for variation in judging the boil and adds a safety margin. It notes that at about 4,900 m water boils at roughly 83 degrees C, which is still well above the temperature needed to inactivate the pathogens that cause traveller's diarrhoea. If fuel is scarce, heat water until small bubbles rise from the bottom, remove it from the heat and keep the container covered for 30 minutes. Boiling is reliable but needs fuel and time, which is why it is what lodge kitchens do and what camping crews do for you.

Filters. Microfilters have pores under 1 micron. The CDC says they remove bacteria and protozoa such as Cryptosporidium and Giardia but do not reliably remove viruses. Ultrafilters with pores of about 0.01 micron should also remove viruses. A device that the US Environmental Protection Agency registers as a "purifier" must show at least 99.9999 per cent removal of bacteria, 99.99 per cent of viruses and 99.9 per cent of Cryptosporidium or Giardia, though the EPA does not independently verify these claims. If you use a microfilter, the CDC says you can treat the filtered water with chlorine to address viruses.

Chlorine. Household bleach at a dose of one to two drops per litre of clear water is effective against bacteria and viruses and against Giardia with longer contact times, but the CDC says it is poorly effective against Cryptosporidium, so filtration is needed for that parasite.

Chlorine dioxide. The CDC says chlorine dioxide is effective against all waterborne pathogens, including Cryptosporidium, at practical doses. The catch is time: tablets and drops need several hours of contact in some cases, and they leave no lasting residual in the water.

Iodine. Iodine has similar contact-time needs to chlorine and is also poorly effective against Cryptosporidium. The CDC says WHO recommends using it for no more than a few weeks, and says to avoid it if you have unstable thyroid disease or an iodine allergy; pregnant travellers should avoid prolonged use.

Cloudy water. The CDC says cloudy water needs higher chemical concentrations or longer contact times, and that you should clarify it first, for example by coagulation-flocculation. On the trail, letting silty water settle and pouring it through a clean cloth before treatment is the simple version of the same idea.

The CDC's combination advice is simple. For clear surface water near human or animal activity, use heat or UV alone, or a microfilter plus a halogen (in either order), or an ultrafilter. For cloudy water, clarify first and then add a second step.

A comparison you can use

The table summarises the CDC chapter; read the source for contact times and doses, which depend on the product.

Choosing for your kind of trek

Teahouse routes such as Annapurna Base Camp and Everest Base Camp. Lodges sell boiled or filtered water, and tea and hot water come from the kitchen. The easiest plan is to buy boiled water in the evening for the next day's bottles and add a chemical step or a filter for refills on the way. The Annapurna region page mentions refill stations where available, which lets you fill a bottle without buying plastic. Carry a filter or tablets as a backup for the days between villages.

Camping routes such as Upper Dolpo. The cook boils water for drinking, but you will still refill a bottle by day. Ask what the crew does for water, and carry your own treatment for water you collect yourself. Our camping trek gear guide covers the rest of the kit. The Dolpo route page also says to use filtered water and to take rubbish out.

Short hikes from Kathmandu or Pokhara. For a day on the hills, carry water from your hotel in a reusable bottle, and do not drink from streams.

Wherever you are: the water in villages and from taps is not the same as bottled or boiled water. Brush your teeth with treated water, keep ice out of drinks you cannot account for, and wash your hands before eating. Most of the stomach problems that stop a trek come from hands and plates, not from the stream.

Bottles: what to carry

Choose a wide-mouth bottle, because it is easier to fill from a tap, to pour boiling water into and to clean. Plastic bottles made for hot liquids tolerate hot refills; thin disposable bottles do not. Many trekkers carry one or two bottles of about a litre each, plus a collapsible container for camp. Insulated bottles keep water from freezing in the cold and double as a hot-water bottle in a sleeping bag, which is a real comfort at altitude. A bottle with a clear measuring scale makes tablet dosing easier.

Do not rely on a hydration bladder alone in winter: the hose freezes. If you use one, blow the water back into the bladder after drinking and keep the hose inside your jacket.

Plastic: the part we all share

Every bottle of water bought on the trail is carried up by porters, mules or yaks and has to be carried back down or burned. Our Leave No Trace guide covers the cost of that waste, and the Sagarmatha entry guide notes that local rules limit single-use plastic in parts of the park. The Rara route page lists refillable bottles and a filter as the expected approach. The lowest-impact method is simple: refill your own bottle and treat it.

Mistakes that cause trouble

• Treating water once and refilling from the same untreated source. Keep a clean bottle for treated water and a separate one for collecting.
• Dipping a cup or a dirty cap into treated water. Recontamination during storage is a real route in; the CDC notes that filters do not prevent it.
• Trusting a filter you have dropped or frozen. If you think it froze, replace it.
• Using tablets before the contact time ends. Read the packet, set a timer and wait.
• Ignoring taste. Chlorine dioxide tablets leave a faint taste; if you dislike it, a drink mix or tea hides it. Do not skip the treatment because of it.
• Carrying too little. Start each day with enough to cover the stretch to the next lodge, and top up as you can.

Planning with a partner

If you are sharing equipment with a partner, one good filter and a pack of tablets for two can do the work of two sets, but you must stay together when you refill. Agree who carries the filter and what you do if it fails. See what to agree with trekking partners before you book. If you are still looking for someone to trek with, the Annapurna Base Camp partner page and the Everest Base Camp partner page list people planning those routes.

A simple plan

For most first-time teahouse trekkers: a wide-mouth bottle or two, a small microfilter or ultrafilter, a pack of chlorine dioxide tablets as a second step and backup, and the habit of buying boiled water in the evening. For camping trips: tablets and a filter for your own refills, and trust the cook for the rest, after asking how it is done. For all trips: do not drink untreated water, wash your hands, keep your bottle clean and take your waste home. More checklists are in the gear and packing hub.